Rotary drill bits are commonly used for drilling wellbores in earth formations. One type of rotary drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which conventionally includes a plurality of cutting elements secured to a face region of a bit body. The bit body of a rotary drill bit may be formed from steel. Alternatively, a bit body may be fabricated to comprise a composite material. A so-called “infiltration” bit includes a bit body comprising a particle-matrix composite material and is fabricated in a mold using an infiltration process. Recently, pressing and sintering processes have been used to form bit bodies of drill bits and other tools comprising particle-matrix composite materials. Such pressed and sintered bit bodies may be fabricated by pressing (e.g., compacting) and sintering a powder mixture that includes hard particles (e.g., tungsten carbide) and particles of a metal matrix material (e.g., a cobalt-based alloy, an iron-based alloy, or a nickel-based alloy). Typically, a metal blank, comprising a metal alloy, such as a steel alloy, is positioned at least partially within the bit body during formation to facilitate attachment of the bit body to a steel shank.
New particle-matrix composite materials are currently being investigated in an effort to improve the performance and durability of earth-boring rotary drill bits. Examples of such new particle-matrix composite materials are disclosed in, for example, now U.S. patent application Ser. No. 11/272,439, filed Nov. 10, 2005, now U.S. Pat. No. 7,776,256, issued Aug. 17. 2010, U.S. patent application Ser. No. 11/540,912, filed Sep. 29, 2006, now U.S. Pat. No. 7,913,779, issued Mar. 29, 2011. and U.S. patent application Ser. No. 11/593,437, filed Nov. 6, 2006, now U.S. Pat. No. 7,784,567, issued Aug. 31, 2010. the disclosure of each of which is incorporated herein in its entirety by this reference.
Such new particle-matrix composite materials may include matrix materials that have a melting point relatively higher than the melting point of conventional matrix materials used in infiltration processes. By way of example and not limitation, nickel-based alloys, cobalt-based alloys, cobalt and nickel-based alloys, aluminum-based alloys, and titanium-based alloys are being considered for use as matrix materials in new particle-matrix composite materials. Such new matrix materials may have a melting point that is proximate to or higher than the melting points of metal alloys (e.g., steel alloys) conventionally used to form a metal blank, and/or they may be chemically incompatible with such metal alloys conventionally used to form a metal blank. Accordingly, bit bodies that comprise such new particle-matrix composite materials may require melting and/or sintering at temperatures proximate to or higher than the melting points of metal alloys (e.g., steel alloys) conventionally used to form a metal blank.
One alternative to the use of a metal blank is an extension (which is also referred to in the art as a “crossover”). One example of an extension is disclosed in U.S. patent application Ser. No. 12/429,059, filed Apr. 23, 2009 and entitled “Earth-Boring Tools and Components Thereof Including Methods of Attaching at Least One of a Shank and a Nozzle to a Body of an Earth-Boring Tool and Tools and Components Formed by Such Methods,” the disclosure of which is incorporated herein in its entirety by this reference. Such extensions provide a means for attaching a bit body to a steel shank after the bit body has been fully formed. The extension conventionally comprises a metal alloy (e.g., steel alloy) and may be coupled to the bit body via, for example, metal brazing or a threaded connection.
During drilling and as is conventional, solids-laden drilling fluid, or “mud,” is pumped down the wellbore through an internal fluid plenum extending through the drill bit to cool and clean the cutting elements on the bit face and to flush debris removed by the drill bit from the subterranean formation being drilled from the bit face and up the wellbore annulus. The drilling fluid passes through a fluid passageway extending, in part, through the extension. As the drilling fluid is caused to flow through the extension, the drilling fluid may erode the interior surfaces of the extension. This erosion can weaken the extension itself and the connections between the extension, the steel shank and the crown. Consequently, this erosion can cause failure of the drill bit, which, in turn, results in time and money expended to replace or repair the drill bit.